Reports by the IOM have focused national attention on the quality and safety of hospital care. There is increasing but inconsistent evidence of a relationship between hospital nurse staffing and adverse patient outcomes. Efforts to address both the current nursing shortage and to strategize around the much larger shortage projected have been hampered by these inconsistencies, and by gaps in the research evidence. This retrospective historical cohort study addresses these needs by taking advantage of 5-years (2000 - 2004) of the unique data resources of Mayo Clinic and its hospitals, which over the study period include data on 250,000 admissions, 1.5 million patient days, and 4.5 million patient shifts. There are three aims. First, at the patient level of detail, we analyze the frequency of adverse patient outcomes and test the relationship between these outcomes and the gap between targeted and actual hours of nursing care, as well as the mix of RNs, LPNS, and unlicensed patient care assistants and predict which patients are most likely to experience these outcomes. Second, at the unit-shift level of analysis, we test for relationships between the measures of nurse staffing identified above and adverse events that occur on a specific shift and unit. Third, we provide estimates of the incremental increases in total costs for patients who experience the adverse patient outcomes in aim 1. Importantly, we are able to risk adjust these outcomes. As a result, we believe we can determine: a) which patients are most at risk for experiencing a complication related to nurse staffing (both hours per patient day and staff mix), b) whether associations between adverse outcomes, adverse events, and nurse staffing are as strong as reported, and hence, whether they should be relied upon in surveillance systems for monitoring hospital quality; c) a new acuity adjustment metrics for use in analyzing nurse sensitive patient outcomes; and d) outcomes and their associated costs at discharge and within 30 days follow-up. Subsequently, this study can provide heretofore missing evidence needed to define nurse staffing levels associated with patient safety in U.S. hospitals. [unreadable] [unreadable] [unreadable]